As you have figured, the old room thermostat can't simply be disconnected as this would leave the heating wiring 'open circuit' and it would not operate, the integrity of the circuit needs to be maintained. As not all thermostats are the same, some working out will be required.

Manual Horstmann CentaurPlus C27 Thermostat Manual Horstmann CentaurPlus C27 Thermostat

Look at the wiring diagram for your make/mode of thermostat that you are removing. If you don't have the instructions for it, a diagram is usually printed inside the lid. This will show which are the two live switching wires, and which is the neutral wire. (If there is a neutral present. Not all thermostats need or have one fitted) note it says Centaur Plus is volt free, so it seems the centaur plus is same as Hive single channel and standard one is same as Hive duel channel, so I would be hunting on the actual device to see if there is a diagram. Your picture clearly shows a brown sleeve on the blue wire going to 3 so clearly a line not neutral.You also need to look for motorised valves, since the standard method is the programmer supplies the thermostat, and the thermostat supplies the motorised valve and the valve has a micro switch which turns on boiler so boiler can't run until valve is open, if some one has left the old wiring in place you may have a valve working like a relay, you can't assume wired correctly when you have already found a programmer wired as it would have been for a system boiler. The diagram above will allow you to identify what the wires in each terminal of the C27 does. Once identified, the wires are moved to the Nest Heat link terminals that have the same function. lt;p>This is the Horstmann CentaurPlus C27, a two channel programmer which gives independent control over hot water and heating with up to 3 ON/OFF settings for each day of the week. It also has a 1 hour manual boost button for the hot water and an advance button for the central heating.</p>

HORSTMANN CENTAURPLUS C27 PROGRAMMER | Wolseley

With what you propose, you are connecting CH and HW control wires to the same BDR91. For example terminal 3 is a HW control wire and terminal 4 is CH control wire, yet you propose to connect them both to the BDR91 for hot water. The same is true of terminals 1 and 2 if they are actually fitted. You can now disconnect all of the wires going to the old thermostat and remove the cable and thermostat. lt;li>Operating periods per day: 3 per day for Central Heating and 3 per day for Hot Water</li> Once you have transferred the wires from the C27 to the Heat link, then the Heat link (L) terminal also needs linking to the two 'common' terminals (2) and (5) Probably no one has answered you because you have added your post to the bottom of someone else's where it is hidden away and not visible as a new post. For future reference, this is also against the forum rules.Override: 1 hour boost (Hot Water only), 1 hour extension to On period (Hot Water only), instant advance (Central Heating only) lt;li>Override: 1 hour boost (Hot Water only), 1 hour extension to On period (Hot Water only), instant advance (Central Heating only)</li> This is the Horstmann CentaurPlus C27, a two channel programmer which gives independent control over hot water and heating with up to 3 ON/OFF settings for each day of the week. It also has a 1 hour manual boost button for the hot water and an advance button for the central heating.